THE NATURE OF CRACKS
With the heating and cooling of refractory on both masonry and factory-built fireplaces, you can expect some minor cracks to occur....it's just the nature of things. Small hairline cracks in the refractory will almost always be present after repeated firings. When these fine cracks occur, you can rest assured that the homeowner can enjoy the fireplace with absolutely no danger! In fact, there is metal behind the refractory on a factory-built fireplace; therefore, even if the cracks get larger, the fireplace will still operate safely.
WHEN TO REPLACE REFRACTORY
One should consider replacing the refractory when large cracks occur. If the crack is larger than 1/16 inch (thickness of a nickel) the refractory should be changed. Also, if the refractory starts to crumble and fall apart in chunks, you should replace it. Refractory replacement on a factory-built fireplace is a relatively simple task, which is usually executed by simply unscrewing a bracket, removing the original refractory and installing the new panel. Repairs are much more difficult on a masonry fireplace because experienced brick masons must use special materials to patch the cracks...and it's much more expensive.
To prevent the problem from happening in the first place, the homeowner should "condition" the refractory in a new fireplace by building relatively small fires for the first few burns and then gradually working up to larger fires. When adding wood to the fire, one should use caution not to throw logs into the firebox, which may hit and damage the back refractory. In almost all cases, the back refractory shows the most signs of wear and tear because it takes the most abuse from heat and log impact. Very rarely do side refractory panels need to be replaced.
As stated previously in this paper, factory built fireplace refractory is a highly engineered product, which if treated with respect, will give the homeowner many years of problem free service. Small cracks are to be expected. If fine cracks appear, it's not a safety issue! Consider replacing the refractory when large cracks are present, pieces of refractory start to break off or if you wish to change refractory for aesthetic reasons.